The present invention relates generally to a speed-load control for a combustion turbine and more particularly to a controller having the ability to transfer between droop and isochronous modes of control with minimal effect on load balance and frequency.
Two fundamental modes for controlling the speed-load characteristics of a combustion turbine are isochronous control and droop control. A controller operating in the isochronous mode maintains the turbine at a constant speed without regard to turbine load. A controller operating in the droop mode controls turbine speed as a function of turbine load.
A typical power generation system for supplying power to a single power generation grid may include turbines having both droop and isochronous controllers. Such a system typically comprises a main generating unit operating in the isochronous mode to maintain the output electric power at a constant frequency and one or more booster generating units operated in the droop mode. In this arrangement the droop controlled units are adjusted to share the load with the main generating unit so as to assure that the isochronous controller is able to maintain a constant speed. The booster generating units are locked in step with the main generating unit, as if mechanically coupled thereto, so that the speed of the booster generating units is fixed by the constant speed of the main generating unit.
The load level on the booster generating units may be controlled by regulating the fuel supply to the turbine driving the units. Generally, increasing the fuel flow to a turbine operating in the droop mode of control increases the load shared by that generating unit. Assume, for example, that a given power generation grid is serviced by a main generating unit and a single booster generating unit operating in parallel. The main generating unit is controlled isochronously while the booster generating unit is controlled in the droop mode. Assume also that the generating units are in a steady-state condition, sharing the load equally.
An increase in fuel flow to the booster generating unit would tend to increase its speed, but because the booster unit is locked in step with the main unit, it cannot increase speed. Consequently, the booster unit now accepts additional load. If the load level of the given power generation grid remains constant, the additional load accepted by the booster unit must be withdrawn from the main unit, which thereafter would tend to increase speed.
The isochronous controller on the main unit, however, compensates for the load loss by decreasing fuel flow so as to maintain a constant speed. Hence, the increased fuel flow to the booster unit caused a transfer of load from the main unit to the booster unit and a reduction in fuel flow to the main unit.
The need sometimes arises in the operation of such power generation systems to change a generating unit's mode of control from droop to isochronous or from isochronous to droop. This might occur, for example, where a main unit must be shut down for maintenance or inspection or where a booster unit must be operated as a main unit to supply power to an isolated portion of the grid. In such cases it is desirable to have a method of changing from droop to isochronous control or from isochronous to droop control which effects minimal disturbance in the output of the power generation system.
U.S. Pat. No. 3,620,010 describes a system for switching the control of the generating unit between the droop mode and the isochronous mode. Although the system described in the named patent is effective to switch between the two control modes, it does little to minimize disturbance to the power output signal and requires extensive operator interaction to effect a successful change.
Consider, for example, a change from the isochronous mode to the droop mode of control. A typical isochronous controller comprises a proportional and integral controller having a speed feedback loop. The change from isochronous to droop requires that a power (or load) feedback signal be algebraically summed with a speed reference signal of the isochronous controller. This constitutes the fundamental structural distinction between the two modes of control. The system in the named patent effects this change by closing a switch, which introduces the power feedback signal to a summing junction. System transients are minimized by pre-adjusting the level of the power feedback signal to prevent an immediate load shift to a parallel generating unit coupled thereto. Load shifts are thereafter accomplished by operator adjustment of feedback signal levels. No attempt is made to minimize the effect of control mode change upon the power level and frequency of the entire power generation system.
Hence, it appears that the known prior art fails to address the problem of providing a wholly automatic system for switching between droop control and isochronous control of a power generating unit while maintaining a substantially constant power output level and power output frequency.